Hot melt adhesive compositions are solid at room temperature. Upon application of heat, melt to a liquid or fluid state in which molten form they are applied to a substrate. On cooling, the adhesive composition regains its solid form. The hard phase(s) formed upon cooling the adhesive composition impart all of the cohesion (strength, toughness, creep and heat resistance) to the final bond. Hot melt adhesive compositions are thermoplastic and can be heated to a fluid state and cooled to a solid state repeatedly. Hot melt adhesive compositions do not include water or solvents.
Curable or reactive hot melt adhesive compositions are also solid at room temperature and, upon application of heat, melt to a liquid or fluid state in which molten form they are applied to a substrate. On cooling, the adhesive composition regains its solid form. The hard phase(s) formed upon cooling the adhesive composition and prior to curing impart initial or green strength to the bond. The reactive hot melt adhesive composition can cure by a chemical crosslinking reaction upon exposure to suitable conditions such as exposure to moisture. Before curing the adhesive composition remains thermoplastic and can be remelted and resolidified. Once cured, the adhesive composition is in an irreversible solid form and is no longer thermoplastic. Reactive hot melt adhesive compositions are stored in the absence of water to prevent premature crosslinking. The crosslinked adhesive composition provides additional strength, toughness, creep resistance and heat resistance to the final bond. Reactive hot melt adhesive compositions can provide higher strength and heat resistance compared to non-curable hot melt adhesive compositions.
The ability of a reactive hot melt adhesive composition to cool so that the solidified but non-crosslinked composition can quickly bond parts together is called green strength. An adhesive composition that quickly develops green strength is desirable in commercial operations as it allows bonded parts to be further processed quickly. Reactive hot melt adhesive compositions will continue to react with moisture after solidification so that strength of the adhesive bond between parts will continue to rise. A high cured strength is desirable in commercial operations as it allows stressed parts to be bonded.
The majority of reactive hot melts are moisture-curing urethane hot melt compositions. The reactive components of urethane hot melt compositions consist primarily of isocyanate terminated polyurethane prepolymers containing urethane groups and reactive isocyanate groups that react with surface or atmospheric moisture to chain extend and form a new polyurethane polymer. Polyurethane prepolymers are conventionally obtained by reacting diols with diisocyanates. Upon cooling the isocyanate groups in the polyurethane prepolymer react with moisture from the environment to form a crosslinked irreversible solid bond.
Moisture-curing urethane hot melt adhesive compositions have certain disadvantages. One disadvantage is the residual monomer content of polyisocyanates, more particularly the more volatile diisocyanates. Some moisture-curing urethane hot melt adhesive compositions can contain significant amounts of unreacted monomeric diisocyanates. At the hot melt application temperature (typically at 100° C. to 170° C.) monomeric diisocyanates have a considerable vapor pressure and may be partly expelled in gaseous form. The isocyanate vapors may be toxic, irritating and have a sensitizing effect, so that precautionary measures have to be taken in the application process.
Silane reactive hot melt adhesive compositions have been developed to solve these issues common to isocyanate reactive hot melt compositions. Silane reactive hot melt adhesive compositions are also solid at room temperature and, upon application of heat, melt to a liquid or fluid state in which molten form they are applied to a substrate. On cooling, the composition regains its solid form. Silane reactive hot melt adhesive compositions are based on silane modified polymers that comprise moisture reactive silane groups that form siloxane bonds when exposed to moisture such as in the atmosphere. Silane reactive hot melt adhesive compositions offer good cured adhesion and since there is no isocyanate monomer there are no concerns about emission of isocyanate monomer vapor. However, many silane reactive hot melt adhesive compositions develop green strength slower than reactive polyurethane hot melt adhesive compositions.
It is difficult to find one reactive hot melt adhesive composition that has a commercially desirable combination of green strength, cured strength, working life and environmental safety. There remains a need for a silane reactive hot melt adhesive composition that has a desirable combination of properties for commercial use including quick development of green strength, a long working life and high final (cured) strength.